Many useful metallic elements such as precious metals and rare metals are contained in various ores, sludges, catalysts, waste electronic materials and the like, and methods for recovering metallic elements therefrom have been considered.
As the methods for recovering metallic elements such as precious metals and rare metals, an amalgam, process using mercury and a cyanidation process using a cyan-based compound are mainly used.
The smelting of gold by the amalgam process is a method including immersing gold ore in mercury to form gold amalgam, and evaporating the mercury to give gold.
The extraction of gold by the cyanidation process is conducted as follows. Firstly, gold contained in gold ore is solved by using an aqueous sodium cyanide solution to form gold cyanide ion (the following formula (1)). The gold cyanide ion is then reacted with sodium, zinc or the like to precipitate gold (the following formulas (2) and (3)). Alternatively, a solution containing gold cyanide ion is electrolysed to precipitate gold (the following formula (4)).[Chemical Formula 1]2Au+4CN−+(½)O2+H2O→2[Au(CN)2]−+2OH−  (1)[Chemical Formula 2][Au(CN)2]−+2Na→Au+2Na++2CN−  (2)[Chemical Formula 3]2[Au(CN)2]−+Zn→2Au+[Zn(CN)4]2−  (3)[Chemical Formula 4]2[Au(CN)2]−+2OH−→2Au+4CN−+H2O+½O2  (4)
Patent Literature 1 discloses a method for recovering a platinum group from a metal carrier catalyst, which is conducted by using a thiocyanate solution containing an oxidant. This method uses a cyan-based compound, and thus requires a waste water treatment.
The amalgam process and cyanidation process use mercury and a cyan-based compound, but both of these compounds are highly toxic and thus may cause environmental pollution. Therefore, it is necessary to sufficiently reduce an environmental load by using an apparatus for recovering exhaust gas and vapor, an apparatus for treating waste water, and the like. However, the cost associated with the introduction, operation and maintenance of these equipments is high.